The present invention relates generally to a surface mount type semiconductor device and a lead frame used for fabricating the same. More particularly, the present invention relates to downsizing a surface mount type semiconductor device without deteriorating reliability thereof and to realizing a lead frame suitable for such downsizing.
Portable type electronic circuit apparatuses such as a video camera, a notebook type personal computer and the like are being urged to be small in size and light in weight. Thus, it is strongly desired that electronic parts used in these electronic circuit apparatuses such as semiconductor devices and the like are also downsized and thinned. In order to cope with such requirements, electronic parts themselves are downsized. Alternatively, even if the external size of each of the electronic parts is the same or slightly larger than before, electronic elements therein are more highly integrated and, thereby, the electronic parts are made substantially small in size and light in weight.
FIG. 14 is a partial perspective view illustrating a conventional lead frame used for fabricating a semiconductor device. The lead frame 105 shown in FIG. 14 is used for fabricating a power semiconductor device which emits much heat when operated.
The lead frame 105 shown in FIG. 14 comprises a pair of band shaped members, that is, first and second band shaped members 101 and 102 disposed parallel to each other. The width of the first band shaped member 101 is smaller than that of the second band shaped member 102. In the second band shaped member 102, holes or perforations 102a for transferring the lead frame 105 are formed at intervals of a predetermined constant length. The lead frame 105 further comprises island portions or heat sinks 103, and leads 104. The heat sinks 103 are disposed outside the first band shaped member 101, that is, on the opposite side of the second band shaped member 102, with predetermined spaces therebetween. The leads 104 comprise lead sets each of which has mutually parallel three leads 104a, 104b and 104c. A large number of lead sets of the leads 104 extend from an edge portion of the second band shaped member 102 beyond the first band shaped member 101. Therefore, the first band shaped member 101 is coupled with the second band shaped member 102 via the leads 104. In each lead set of the leads 104, the center lead 104a is coupled, at an edge portion thereof, with an edge portion of the heat sink 103. End portions of other leads in each lead set are not coupled with the heat sink, but are located near the edge portion of the heat sink 103.
With reference to the drawings, an explanation will be made on a method of manufacturing a conventional power semiconductor device. FIG. 15 is a side cross sectional view of a conventional power semiconductor device fabricated by using the lead frame 105 shown in FIG. 14, and FIG. 16 is a top perspective view of the semiconductor device. For the sake of easy understanding, FIG. 16 shows a structure of a portion within an encapsulation resin by using perspective representation.
First, a semiconductor pellet 107 is mounted on the heat sink 103 by using a solder 106. Then, electrodes (not shown in the drawing) on the semiconductor pellet 107 and the leads 104b and 104c are electrically coupled via wires 108a and 108b, respectively. The wire 108a through which a main current flows is constituted of a thick wire. The main portion on the heat sink 103 including the semiconductor pellet 107 is coated with an encapsulation resin 109. In this case, the back surface of the heat sink 103 is exposed from the encapsulation resin 109. Also, as shown in FIG. 16, the lead 104a is disposed in a concave portion 109a of the encapsulation resin 109. Therefore, the surface of the encapsulation resin 109 from which the lead 104a coupled with the heat sink 103 comes out is recessed from the surface of the encapsulation resin 109 from which other leads 104b and 104c come out. Thereby, creepage distances between the lead 104a and the leads 104b and between the lead 104a and 104c can be elongated, and it is possible to assure a safe operation of the semiconductor device at a high voltage.
After encapsulation by the encapsulation resin 109 is completed, unnecessary portions of the first and second band shaped members 101 and 102 of the lead frame 105 which connect the leads 104 are cut and removed. Thereby, the leads 104 are separated and the semiconductor device shown in FIG. 16 is completed.
Also, the center lead 104a is cut within the concave portion 109a of the encapsulation resin 109. Each of the leads 104b and 104c is bent into a crank shape near the encapsulation resin 109. Thereby, end portions of the leads 104b and 104c are made coplanar with the exposed surface of the heat sink 103. FIG. 17 is a side cross sectional view showing a conventional surface mount type power semiconductor device which is manufactured in this way. In the semiconductor device shown in FIG. 17, it is possible to directly solder the heat sink 103 and the leads 104b and 104c to conductive islands of a wiring substrate not shown in the drawing. Therefore, it is possible to lower the height of the semiconductor device mounted on the wiring substrate. Semiconductor devices of this type are disclosed, for example, in Japanese utility model laid-open publication No. 62-188149, Japanese patent laid-open publication No. 4-340264, Japanese patent laid-open publication No. 5-283574 and the like.
In the above-mentioned conventional semiconductor device, it is possible to lower the height thereof. However, since the leads 104b and 104c protrude from the encapsulation resin 109, it is impossible to sufficiently reduce the mounting area of the semiconductor device.
FIG. 18 is a side cross sectional view illustrating another conventional surface mount type power semiconductor device which can obviate the above-mentioned disadvantage. FIG. 19 is a bottom view of the semiconductor device of FIG. 18. In FIG. 18 and FIG. 19, like reference numerals are used to designate identical or corresponding parts to those of the conventional semiconductor device of FIG. 17, and detailed description thereof is omitted here. In the semiconductor device shown in FIGS. 18 and 19, portions of leads 104b and 104c near a heat sink 103 are made coplanar with a surface of the heat sink 103. Also, at the bottom surface of the semiconductor device, portions of the leads 104b and 104c together with the heat sink 103 are exposed from an encapsulation resin 109. Leads 104a, 104b and 104c coming out from the encapsulation resin 109 are cut in the proximity of the encapsulation resin 109. By using this structure, it is possible to further downsize the semiconductor device.
In the semiconductor device having the structure shown in FIG. 18, the area of the heat sink 103 is made as large as possible so that good heat dissipating ability can be obtained. However, in this semiconductor device, it is necessary that the leads 104b and 104c are disposed apart from the lead 104a. Therefore, the areas of electrode portions of the leads 104b and 104c exposed from the encapsulation resin 109 at the bottom surface of the semiconductor device must be relatively small with respect to the exposed area of the heat sink 103.
When the semiconductor device having this structure is soldered on conductive land portions of a wiring substrate, the semiconductor device floats on melted solders and becomes unstable. Therefore, there was a possibility that the semiconductor device rotates or moves from a predetermined mounting location of the semiconductor device.
Further, the heat sink 103 and the leads 104b and 104c are disposed coplanar with each other. Therefore, when the thickness of the heat sink 103 is made larger than that of the leads 104b and 104c to obtain good heat dissipating ability, the difference of height between a semiconductor pellet 107 and the leads 104b and 104c becomes large.
The outer size of the heat sink 103 is reduced as small as possible to downsize the semiconductor device. On the other hand, the outer size of the semiconductor pellet 107 is determined from operating characteristics such as an operating current, an operating power and the like, and reduction of the outer size of the semiconductor pellet 107 is limited. Therefore, the outer sizes of the heat sink 103 and the semiconductor pellet 107 become close to each other. As a result thereof, it is impossible to keep an enough distance from an outside edge portion of the semiconductor pellet 107 to an outside edge portion of the heat sink 103.
Further, in the power semiconductor device, a main current flows from the heat sink 103 through the semiconductor pellet 107 to a surface electrode of the semiconductor pellet 107, and then reaches the lead 104b via the wire 108a. The current passed through the semiconductor pellet 107 reaches the wire 108a via the thin surface electrode of the semiconductor pellet 107. Therefore, if the wire 108a is connected to a peripheral portion of the electrode, on-resistance of the semiconductor device becomes large and an operation at high current is restrained. In order to avoid such restraint, as the wire 108a, a plurality of separate wires are used to parallel couple between the electrode of the semiconductor pellet 107 and the lead 104b. 
Therefore, it is impossible to couple the surface electrode of the semiconductor pellet 107 and the lead 104b by using the wire 108a via the shortest distance. There is a possibility that a middle portion of the wire 108a bends and approaches a peripheral corner portion of the semiconductor pellet 107. This deteriorates withstand voltage characteristics of the semiconductor device, and at worst causes short circuit between the wire 108a and the semiconductor pellet 107.
In the conventional lead frame 105, the lead 104a which supports the heat sink 103 at one end thereof has a relatively long portion from the heat sink 103 to the first band shaped member 101. Therefore, the lead 104a bends easily during a manufacturing process of the semiconductor device. Especially, when the thickness of the heat sink 103 is large, there is a possibility that the lead 104a bends and deforms at its middle portion due to the weight of the heat sink 103.
In order to avoid such disadvantage, it may be possible to enlarge the width of the lead 104a to increase the strength of the lead 104a. When the width of the lead 104a is enlarged, however, the spaces between the lead 104a and other leads 104b and 104c as electrodes become short and withstand voltage characteristics of the semiconductor device are deteriorated. Further, the width of the lead 104a cannot be sufficiently large because of the restriction of the width of the electrode portion.
Further, it may be possible to use a conductor tape having a relatively large width in place of the wire 108a and to realize a low on-resistance thereby. However, in such case, when the thickness of the encapsulation resin 109 is decreased to thin down the semiconductor device, it becomes impossible to completely fill the electrode portion with the resin because the conductor tape becomes a hindrance. Thus, voids are formed in the encapsulation resin 109. Even if such voids do not appear at the outside surface of the encapsulation resin 109, a substantial thickness of the encapsulation resin 109 becomes small. This deteriorates moisture resistance of the semiconductor device and deteriorates bonding strength between the electrode portion and the encapsulation resin 109. Therefore, reliability of the semiconductor device is greatly deteriorated.
Therefore, it is an object of the present invention to provide a surface mount type semiconductor device and a lead frame used for manufacturing the same in which the abovementioned disadvantages of the conventional technology can be obviated.
It is another object of the present invention to provide a surface mount type semiconductor device and a lead frame used for manufacturing the same in which downsizing of the semiconductor device can be attained without deteriorating reliability.
It is still another object of the present invention to provide a surface mount type semiconductor device which can be mounted precisely on a predetermined location of a wiring substrate, and a lead frame used for manufacturing such semiconductor device.
It is still another object of the present invention to provide a surface mount type semiconductor device which has an improved withstand voltage, and a lead frame used for manufacturing such semiconductor device.
It is still another object of the present invention to provide a surface mount type semiconductor device which has an improved moisture resistance, and a lead frame used for manufacturing such semiconductor device.
According to an aspect of the present invention, there is provided a lead frame used for manufacturing semiconductor devices, the lead frame comprising: first and second band shaped members disposed parallel to each other; a plurality of island portions for mounting semiconductor pellets thereon respectively, wherein the plurality of island portions are disposed at predetermined intervals between the first and second band shaped members, and wherein a first end portion of each of the island portions is connected to the first band shaped member; a coupling strip provided for each of the island portions, wherein the coupling strip is disposed between each of the island portions and the second band shaped member, wherein a first end portion of the coupling strip is connected to a second end portion of each of the island portions, and wherein a second end portion of the coupling strip is connected to the second band shaped member; and at least one electrode portion which is provided for each of the island portions and which is to be electrically coupled with a corresponding electrode of the semiconductor pellet mounted on each of the island portions, wherein the at least one electrode portion is disposed between each of the island portions and the second band shaped member, wherein a first end portion of the at least one electrode portion is connected to the second band shaped member, and wherein a second end portion of the at least one electrode portion is opposed to the second end portion of each of the island portions but is not connected to the second end portion of each of the island portions.
In this case, it is preferable that the width of the coupling strip is smallest at the first end portion and becomes gradually larger toward the second end portion, and wherein the width of the at least one electrode portion is larger than the width of the coupling strip.
It is also preferable that the at least one electrode portion comprises two electrode portions corresponding to each of the island portions, the first end portions of the two electrode portions are connected to the second band shaped member on both sides of the coupling strip.
It is further preferable that the first band shaped member has perforations for transferring the lead frame.
It is advantageous that the width of the second band shaped member is smaller than the width of the first band shaped member.
According to another aspect of the present invention, there is provided a lead frame used for manufacturing semiconductor devices, the lead frame comprising: first and second half frame structure portions; and a plurality of bridge members for connecting the first and second half frame structure portions; wherein each of the first and second half frame structure portions comprises: first and second band shaped members disposed parallel to each other; a plurality of island portions for mounting semiconductor pellets thereon respectively, wherein the plurality of island portions are disposed at predetermined intervals between the first and second band shaped members, and wherein a first end portion of each of the island portions is connected to the first band shaped member; a coupling strip provided for each of the island portions, wherein the coupling strip is disposed between each of the island portions and the second band shaped member, wherein a first end portion of the coupling strip is connected to a second end portion of each of the island portions, and wherein a second end portion of the coupling strip is connected to the second band shaped member; and at least one electrode portion which is provided for each of the island portions and which is to be electrically coupled with a corresponding electrode of the semiconductor pellet mounted on each of the island portions, wherein the at least one electrode portion is disposed between each of the island portions and the second band shaped member, wherein a first end portion of the at least one electrode portion is connected to the second band shaped member, and wherein a second end portion of the at least one electrode portion is opposed to the second end portion of each of the island portions but is not connected to the second end portion of each of the island portions; and wherein the first and second half frame structure portions are disposed symmetrically such that the second band shaped members are located inside, and wherein the bridge members are connected to the second band shaped member of the first half frame structure portion and to the second band shaped member of the second half frame structure portion, thereby connecting the second band shaped members of the first and second half frame structure portions together.
It is preferable that each of the bridge members has a perforation for transferring the lead frame.
It is also preferable that, in each of the half frame structure portions, there are provided two electrode portions corresponding to each of the island portions, the first end portions of the two electrode portions are connected to the second band shaped member on both sides of the coupling strip.
It is further preferable that each of the bridge members is located between the coupling strip of the first half frame structure portion and the coupling strip of the second half frame structure portion, wherein a portion of the second band shaped member of the first half frame structure portion connecting to each of the bridge members does not overlap a portion of the second band shaped member of the first half frame structure portion connecting to the electrode portion, and wherein a portion of the second band shaped member of the second half frame structure portion connecting to each of the bridge members does not overlap a portion of the second band shaped member of the second half frame structure portion connecting to the electrode portion.
It is advantageous that each of the perforations is located between the coupling strip of the first half frame structure portion and the coupling strip of the second half frame structure portion.
It is also advantageous that the width of the coupling strip is smallest at the first end portion and becomes gradually larger toward the second end portion, and wherein the width of the at least one electrode portion is larger than the width of the coupling strip.
It is further advantageous that the width of the second band shaped member is smaller than the width of the first band shaped member.
According to still another aspect of the present invention, there is provided a surface mount type semiconductor device comprising: a conductive island portion; a semiconductor pellet mounted on the top surface of the conductive island portion, the semiconductor pellet having at least one electrode formed on the semiconductor pellet; a conductive strip portion, one end of which connects to a portion of a first end portion of the conductive island portion; at least one electrode portion which is electrically coupled with corresponding one of the at least one electrode of the semiconductor pellet, each of the at least one electrode portion does not connect to the conductive island portion; and an encapsulation resin which covers and unifies the semiconductor pellet, the electrode portion, the conductive island portion and the conductive strip portion; wherein, at the bottom surface of the semiconductor device, the bottom surface of the conductive island portion and a portion of the bottom surface of each of the at least one electrode portion are exposed from the encapsulation resin; wherein the portion of the bottom surface of each of the at least one electrode portion exposed from the encapsulation resin and the bottom surface of the conductive island portion are coplanar with each other; wherein, at a side surface of the encapsulation resin, a first end portion of each of the at least one electrode portion and a second end portion of the conductive strip portion are exposed from the encapsulation resin; wherein the width of each of the at least one electrode portion is larger than the width of the conductive strip portion; and wherein a second end portion of each of the at least one electrode portion is raised from the exposed surface of each of the at least one electrode and is located inside the encapsulation resin.
In this case, it is preferable that the conductive island portion functions as a heat sink.
It is also preferable that the at least one electrode portion comprises a plurality of electrode portions.
It is further preferable that the at least one electrode portion comprises two electrode portions.
It is advantageous that the encapsulation resin has a concave portion formed in a side surface of the encapsulation resin, a second end portion of the conductive strip portion protrudes from the concave portion, and first end portions of the two electrode portions protrude from both side of the concave portion in the side surface of the encapsulation resin.
It is also advantageous that the at least one electrode is electrically coupled with the at least one electrode on the semiconductor pellet by wire bonding.
It is further advantageous that, among the at least one electrode on the semiconductor pellet, an electrode through which a main current is to flow is electrically coupled with a corresponding one of the at least one electrode portion via a conductive tape.